To measure a multiphase current flowing through an inverter or the like, it is difficult to accurately measure a current in each phase using a small apparatus that uses a magnetic sensor. A technique of combining a measurement target conductor, a magnetic core, and a magnetic sensor together has often been used to accurately measure the current in each phase without suffering any adverse effect of a magnetic field generated by a current in different phase. A technique has often been used which involves placing a printed circuit board with a signal processing integrated circuit and a magnetic sensor installed thereon in a gap in the core. In this case, the magnetic core is integrated with a printed circuit board. This disadvantageously prevents a reduction in the size of the apparatus. Further, if the circuit board with the magnetic sensor installed thereon is placed in the gap or near the measurement target conductor, an induced electromotive force is exerted in a wiring loop formed by a signal wire between the magnetic sensor and the signal processing circuit. The voltage level of the induced electromotive force can be easily increased above that of an output from the magnetic sensor. As means for correcting this, the signal processing circuit has been improved or corrections have been made by, for example, winding a coil around a core. However, these techniques complicate the circuit or the configuration, making the size reduction difficult.
JP2002-243766A and JP2001-153895A describe a current sensor that detects a magnetic field generated by a current flowing through a measurement target to determine a current value. Further, JP-6-201732A describes a current sensor having a magnetic sensor and a signal processing circuit both mounted on the same circuit board, which is inserted into a gap in a magnetic core.
With the current sensor described in JP2002-243766A and JP2001-153895A, when a measurement target current flows at a frequency of several 100 Hz or higher, in addition to a signal generated by the magnetic sensor, an induced electromotive force caused by the measurement target current is exerted in the wiring between the magnetic sensor and a detection circuit or in the wiring loop in the detection circuit. This disadvantageously prevents accurate measurements. Furthermore, when a multiphase current is to be detected, a magnetic field from a nearby current wire is not uniform and can thus not appropriately be eliminated.
According to JP6-201732A, when the frequency band of the current to be measured increases, it is necessary to minimize the wiring loop area between the magnetic sensors and the signal processing circuit while keeping the distance between the magnetic sensors and the signal processing circuit at a certain value or greater in order to reduce the adverse effect of the induced electromotive force.
An object of the present invention is to provide a current measuring apparatus having an improved structure to inhibit the possible adverse effect of a current in a different phase and the possible generation of an induced electromotive force caused by a measurement target current, enabling the measurement target current to be accurately detected even with the small size of the apparatus.